Mastery of witty leaders in the newspapers (7)
I believe the answer is:
prowess
'mastery' is the definition.
(thesaurus)
'of witty leaders in the newspapers' is the wordplay.
'leaders' says to take the initial letters.
'in the' is an insertion indicator.
'newspapers' becomes 'press' (I've seen this before).
The first letters of 'of witty' is 'ow'.
'ow' put inside 'press' is 'PROWESS'.
(Other definitions for prowess that I've seen before include "Skill in a particular field" , "Ability, expertise" , "'Skill, expertise (7)'" , "Bravery in battle" , "Skill, expertise; bravery" .)